Multiple alloy rotor and method therefor

ABSTRACT

A process for producing a rotor, the rotor formed thereby, as well as turbines in which such a rotor is installed. The rotor is formed by casting an ingot to have first and second regions formed of different alloys that intermix during casting to define a transition zone therebetween. The ingot is forged to yield a rotor forging that contains axially-aligned first and second alloy regions and a transition zone therebetween. The effects of the transition zone can be mitigated by modeling the transition zone and then off-center machining the forging so that the axis of rotation of the machined monolithic rotor is more centrally located with respect to the transition zone.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part patent application of co-pending UnitedStates patent application Serial No. {Attorney Docket No. 132847}, filedJun. 18, 2003.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention generally relates to turbine rotors, such as thoseused in steam turbines, gas turbine engines, and jet engines. Moreparticularly, this invention relates to a rotor and method of producinga monolithic rotor containing two or more alloys within separate regionsof the rotor resulting in a transition zone between different alloyregions, and to a method of determining the shape of the transition zoneto enable the final machined geometry of the rotor to reduce any thermalinstability attributable to the presence of the transition zone.

2. Description of the Related Art

Rotors used in steam turbines, gas turbines and jet engines typicallyexperience a range of operating conditions along their lengths. Thedifferent operating conditions complicate the selection of a suitablerotor material and the manufacturing of the rotor because a materialoptimized to satisfy one operating condition may not be optimal formeeting another operating condition. For instance, the inlet and exhaustareas of a steam turbine rotor have different material propertyrequirements. The high temperature inlet region typically requires amaterial with high creep rupture strength but only moderate toughness.The exhaust area, on the other hand, does not demand the same level ofhigh temperature creep strength, but suitable materials typically musthave very high toughness because of the high loads imposed by longturbine blades used in the exhaust area.

Because a monolithic rotor (i.e., a rotor that is not an assembly) of asingle chemistry cannot meet the property requirements of each of theLP, IP and HP stages for the reasons discussed above, rotors constructedby assembling segments of different chemistries are widely used. Forexample, large steam turbines typically have a bolted construction madeup of separate rotors contained in separate shells or hoods for use indifferent sections of the turbine. More recently, the steam turbineindustry has favored CrMoV low alloy steels for use in the HP stage andNiCrMoV for use in the LP stage, though NiMoV low alloy steels have alsobeen widely used as materials for the various stages. Smaller steamturbines may make use of a mid-span coupling to bolt high and lowtemperature components together within one shell. Finally, rotors forgas turbines and jet engines are often constructed by bolting a seriesof disks and shafts together. While rotors having a bolted constructionare widely used, they suffer from several disadvantages includingincreased numbers of parts, increased assembly requirements, increasedlength of the rotor assembly, and more balance complexity.

Another method of combining different materials in a single rotor is toweld together rotor segments formed of dissimilar materials, formingwhat may be termed a multiple alloy rotor (MAR). However, a welded rotorconstruction also has disadvantages, such as high investment costs forthe welding equipment, additional production costs for weld preparationand welding, long production times to produce, inspect and upgrade theweld, and increased cost and production time caused by the need for postweld heat treatment. The strength of rotors having a welded constructioncan also be limited due to a need to maintain a low carbon content inthe weld, and the propensity for high numbers of small non-metallicinclusions that reduce load carrying capability.

The capability of producing a monolithic MAR would address theabove-noted shortcomings of assembled MAR's. Furthermore, monolithicMAR's would be particularly well suited for meeting the demand forhigher efficiency steam turbines whose requirements include low pressure(LP), intermediate pressure (IP) and high pressure (HP) stages (orcombinations thereof) with additional stages in areas normally occupiedby couplings. Consumable electrode remelting techniques such aselectro-slag remelting (ESR) and vacuum arc remelting (VAR) methodsoffer flexibility for producing components that contain alloycombinations, and therefore has been considered for producing monolithicMAR's. As an example, U.S. Pat. No. 6,350,325 to Ewald et al. disclosesan ESR method of producing a dual alloy rotor from 12Cr-type alloys thathave different levels of alloying constituents, but are sufficientlyclose in composition so as to have substantially identical austenitizingtemperatures. Ewald et al. also disclose that, because the alloys havesimilar compositions, problems can be avoided that are associated withmixing of alloys having significantly different material properties,which results in the formation of a transition zone (TZ) between regionsof the rotor formed by the different alloys.

One such problem is thermal stability arising from the massive size of arotating rotor supported by bearings at each end of the rotor. Whensupported in this manner, a rotor behaves as a simply supported beamstructure and will deflect in reaction to the centrifugal load alwayspresent at operational conditions, with the largest deflection beingnear the center of the rotor. Because of the inherent asymmetry of thetransition zone within a MAR rotor, deflection significantly increaseswhen the rotor is at its elevated operating temperatures. As the rotorrotates about its bent centerline, the rotor material is subjected tohigh cycle fatigue as a result of being in tension and then incompression with each rotation. Consequently, reducing deflection byminimizing material asymmetry is necessary to maximize the life of a MARrotor and the turbine in which it is installed. One solution is to limitthe rotor to alloys with similar compositions. However, this restrictionlimits the ability to optimize the compositions of the LP, IP and HProtor sections for their operating environments and cost. For example,such a limitation has dissuaded the manufacture of a monolithic MARwhose HP stage is formed of CrMoV and its LP stage is formed of NiCrMoV.Therefore, it would be desirable if an improved process were availablefor producing turbine rotors of different alloy compositions.

SUMMARY OF INVENTION

The present invention provides a process for producing a rotor, therotor formed thereby, as well as turbines in which such a rotor isinstalled. The rotor is formed by machining a single rotor forging tohave at least two axially-aligned rotor regions and a transition zonetherebetween. According to a particular aspect of the invention, therotor is a monolithic multiple alloy rotor (MAR), wherein the rotorregions are formed of different alloys and the transition zone has acomposition that differs from and varies between the rotor regions.

The process of this invention involves casting a multiple-alloy ingothaving at least first and second ingot regions axially aligned withinthe ingot, with the first and second ingot regions being formed ofdifferent alloys so that intermixing occurs during casting to define atransition zone therebetween having a composition that differs from andvaries between the first and second ingot regions. The ingot is thenforged to produce a rotor forging containing first and second forgingregions and a transition zone therebetween corresponding to the firstand second ingot regions and the transition zone of the ingot, i.e., thefirst and second forging regions are formed of the different alloys andthe transition zone of the rotor forging has a composition that differsfrom and varies between the first and second forging regions. The firstand second forging regions and the transition region therebetween areaxially aligned along a geometric centerline of the rotor forging.Following heat treatment, the rotor forging is machined to produce amachined rotor containing first and second rotor regions and atransition zone therebetween corresponding to the first and secondforging regions and the transition zone of the rotor forging, i.e., thefirst and second rotor regions are also formed of the different alloysand the composition of the transition zone within the machined rotordiffers from and varies between the first and second rotor regions.

According to one aspect of the invention, the transition zone within therotor forging is asymmetrical about the geometric centerline of therotor forging. This asymmetry of the transition zone may be attributableto asymmetry of the transition zone within the ingot and/or as a resultof the forging operation, the latter of which always degrades thesymmetry of the transition zone to some degree. Because the materialproperties of the rotor vary with the composition of the transitionzone, asymmetry of the transition zone causes asymmetrical variations inthe mechanical and physical properties of the rotor, which if notmitigated promotes bending of the rotor and thermal instability duringoperation. According to the invention, the asymmetry of the transitionzone is mitigated by producing a three-dimensional approximation of theshape of the transition zone, and then using the three-dimensionalapproximation to predict deflection of the geometric centerline of therotor forging if the forging were to be heated to an elevatedtemperature. The three-dimensional approximation of the shape of thetransition zone can be produced by combining measurements of thechemistry on the surface of the rotor with either knowledge of the shapeof the solidified transition zone melt pool after forging or byultrasonic inspection to identify the three-dimensional shape of thetransition zone. With this knowledge, the rotor forging can be machinedso that its axis of rotation is more centrally located with respect tothe transition zone of the rotor, and therefore more centrally locatedwith respect to the material properties of the rotor. As an optionaladditional step following rough machining of the rotor forging, astandard heat indication test can be performed on the rotor to measureits tendency to bend when heated and, if the tendency is larger thandesired, the results of the heat indication test can be used to optimizethe final machining of the rotor to reduce the bending tendency.

In view of the above, it can be seen that a significant advantage ofthis invention is that a multiple alloy rotor can be produced by castingand forging without the limitations previously placed on the alloys usedto form such rotors. In particular, the present invention permits theuse of dissimilar alloys such as NiCrMoV and CrMoV alloys, which formtransition zones that, using prior art processing approaches, wouldresult in a rotor that exhibits unacceptable thermal instability. Theinvention is able to overcome this restriction with a process thatreduces the asymmetrical property variations attributable to thetransition zone, thereby permitting different sections of the rotor tobe formed of alloys that have the potential for optimizing the differentrotor sections (e.g., HP, IP, and LP) for their operating environments.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically represents processing steps performed to produce amultiple alloy rotor forging having a transition zone in accordance witha preferred embodiment of this invention.

FIG. 2 schematically represents a partial cross-sectional view of arotor forging formed by the process of FIG. 1, wherein chemical analysisis performed at points within and adjacent the transition zone inaccordance with an embodiment of the invention.

FIG. 3 shows a rotor forging formed by the process of FIG. 1, whereinmetallurgical characteristics within and adjacent the transition zoneare determined by ultrasonic inspection in accordance with anotherembodiment of the invention.

FIG. 4 is a chart showing data collected from chemical analysis of arotor forging in accordance with the embodiment of FIG. 2.

FIG. 5 is a two-dimensional plot generated from the data of FIG. 4 andidentifying the boundaries of regions within the transition zone havingdifferent nickel contents.

FIG. 6 represents a two-dimensional plot of one of the boundaries ofFIG. 5 measured at a single cross-section and rotated to variouscircumferential positions to illustrate transformations used to modelthe asymmetry of the transition zone.

FIG. 7 is a graph comparing the predicted and measured centerlinedeflection along the axis of a rotor as a result of the asymmetry of thetransition zone within the rotor, in which the predicted centerlinedeflection was based on a chemical analysis performed in accordance withthe embodiment of FIGS. 2 and 4 through 6.

FIG. 8 is a chart showing data collected from an ultrasonic scan of thetransition zone at one axial position of a rotor forging in accordancewith the embodiment of FIG. 3.

FIG. 9 is a three-dimensional plot generated using multiple ultrasonicscans similar to FIG. 8 and taken at multiple axial positions of a rotorforging.

FIG. 10 represents an analytical technique for generating athree-dimensional mathematical model of the transition zone using thedata of FIGS. 8 and 9.

FIG. 11 is a graph comparing the predicted and measured centerlinedeflection along the axis of a rotor as a result of the asymmetry of thetransition zone within the rotor, in which the predicted centerlinedeflection was based on an ultrasonic inspection performed in accordancewith the embodiment of FIGS. 3 and 8 through 10.

FIG. 12 represents an analytical technique for transforming thecoordinate system of a rotor to a new coordinate system whose center ismore centrally located within the transition zone to reduce rotorcenterline deflection.

DETAILED DESCRIPTION

The present invention generally pertains to the production of a multiplealloy rotor (MAR) using a casting technique, preferably a consumableelectrode remelting technique such as electro-slag remelting (ESR) orvacuum arc remelting (VAR), and various combinations of alloychemistries to achieve properties suitable for different regions of therotor, such as the high, intermediate and low pressure turbine stages ofan advanced power generation steam turbine, gas turbine, or aircraftengine. In so doing, a transition zone is present within the rotorbetween regions that have substantially uniform compositions that differfrom region to region.

FIG. 1 represents such a process, by which a monolithic (one-piece)rotor forging 10 is produced by ESR in accordance with the invention.The forging 10 (of which only half is depicted in FIG. 1) is representedas having been rough machined to define high pressure (HP), intermediatepressure (IP), and low pressure (LP) sections 12, 14 and 16,respectively, along a geometric centerline 18 of the forging 10. Theforging 10 is also represented as having a transition zone 20 betweentwo regions 22 and 24 of the forging 10 formed of an IP/HP Alloy and anLP Alloy, respectively. FIG. 1 shows the LP alloy region 24 and thetransition zone 20 generally limited to the LP section 16 of the forging10, while the IP/HP alloy region 22 is generally limited to the HP andIP sections 12 and 14 of the forging 10. Various other configurationsare possible, e.g., the transition zone 20 could extend into the IPsection 14 of the forging 10. The alloys are preferably selected to havemechanical and physical properties that are optimized for theirrespective locations within a rotor that will be subsequently machinedfrom the forging 10. As such, the IP/HP and LP alloy compositions aredifferent but substantially uniform within their respective regions 22and 24. In contrast, the composition within the transition zone 20varies between each of its boundaries with the regions 22 and 24 in viewof the compositional differences between the LP and IP/HP alloys and theprocess by which the forging 10 is cast.

As with conventional ESR techniques, FIG. 1 shows an electrode 30suspended over a melt pool 32 contained in a chilled crucible 34. In theESR process, resistance heating between the electrode 30 and molten slag(not shown) on the surface of the pool 32 causes melting of theelectrode 30. During melting, molten droplets of the electrode 30 dropthrough the slag, where they solidify to produce a directionallysolidified ingot 50 having discrete regions 60, 62 and 64 along itsaxial length. The electrode 30 is depicted as comprising least threedistinct regions 36, 38 and 40. The lower and upper regions 36 and 40 ofthe electrode 30 are formed of alloys corresponding to the IP/HP and LPalloys desired for the regions 22 and 24 of the forging 10. Inaccordance with an optional aspect of the invention, the intermediateregion 38 of the electrode 30 may have a composition that differs fromthe other two electrode regions 36 and 40 for the purpose of tailoringthe composition of the transition zone 20 within the forging 10. Theregions 36, 38 and 40 of the electrode 30 can be produced using air orvacuum melting processes, followed by the use of a welding process or anESR or VAR process to join the regions 36, 38 and 40 end-to-end.Alternatively, a separate piece of each alloy could be meltedindividually and in sequence to form the ingot 30.

As the process of this invention is represented in FIG. 1, the lowerregion 62 of the ingot is represented as being formed by the IP/HP alloy(lower region 36) of the electrode 30, and the upper region 64 isindicated as being formed by the LP alloy (region 40) of the electrode30, though the order of the IP/HP and LP alloys could be reversed fromthat shown. The lower and upper regions 36 and 40 of the electrode 30have lengths appropriate to create the corresponding regions 62 and 64within the ingot 50. The middle region 60 of the ingot 50 is atransition zone between the regions 62 and 64. Regardless of whether theelectrode 30 includes the intermediate region 38, the transition zone 60inherently forms as a result of intermixing of the IP/HP and LP alloysduring the ESR process. If the electrode 30 is formed to include theintermediate region 38, the intermediate region 38 can be formulated tohave a special intermediate chemistry to control the chemistry gradientand/or the width of the transition zone 60 within the ingot 50. However,the shape and size of the transition zone 60 cannot be preciselycontrolled in view of variables in the ESR process. Therefore, thetransition zone 60 may have a shape whose axial boundaries, thoughcorresponding to the shape of the melt pool 32, can be asymmetrical tosome degree about the axis of the ingot 50. The regions 60, 62 and 64 ofthe ingot 50 form the transition zone 20 and the regions 22 and 24,respectively, of the rotor forging 10 forged from the ingot 50. Ofparticular significance to the present invention, any forging operationperformed on the multiple-alloy ingot 50 of this invention willinherently degrade the symmetry of the transition zone 60 within theingot 50, such that the shape of the transition zone 20 within the rotorforging 10 can be significantly asymmetrical relative to the centerline18 of the forging 10.

Various characteristics are required for the different sections 12, 14and 16 of the forging 10 in order to meet the properties required of arotor machined from the forging 10, such as tensile strength, fracturetoughness, rupture strength, thermal stability, and high processcapability (repeatability and reproducibility), as well as cost targets.In order to achieve the mechanical properties desired for the rotor, thechemistries of the multiple alloys of the forging 10 are likely to besufficiently different to require different heat treatment temperaturesand durations, such that a differential heat treatment must be performedon the forging 10 prior to machining. For this purpose, a furnace withmultiple temperature zones is used to provide an appropriate heattreatment temperature for each region 20, 22 and 24 of the rotor forging10. The heat treatment may include a differential temperature for boththe solution or austenitizing treatment and the aging or temperingtreatment of the particular alloys. For steels, a higher temperatureaustenitizing treatment is preferably used if higher creep rupturestrength is desired (e.g., for the high pressure rotor section 12),while relatively lower temperatures are used if higher toughness isneeded (e.g., for the low pressure rotor section 16). Differentialcooling from the solution or austenitizing temperature is alsopreferably used. Rapid cooling can be used to achieve full sectionhardening, to avoid harmful precipitation reactions, and/or to enhancetoughness (e.g., for the low pressure rotor section 16). Slow coolingcan be used to achieve beneficial precipitation reactions, to reducethermal stresses, and/or to enhance creep rupture strength (e.g., forthe high pressure rotor section 12). Particular temperatures, durations,and heating and cooling rates suitable for the forging 10 will depend onthe materials used, and such heat treatment parameters will generally bewithin the capability of one skilled in the art.

Notable commercial alloys that are suitable for use as the LP alloyinclude the conventional NiCrMoV-type low alloy steels andchromium-containing martensitic stainless steels such as the12Cr-3Ni—Mo—V type alloy (M152). Alloys that are suitable for use as theIP/HP alloy of the IP and HP sections 12 and 14 include a variety ofalloys having increasing high temperature capability, such as theconventional CrMoV low alloy steels, 9-14 Cr-type alloys with varyinglevels of Mo, V, W, Nb, B and N, Fe—Ni alloys (such as A286), andnickel-base alloys (such as Alloy 706 or 718). Particularly preferredalloys for the LP section 16 include the following: TABLE 1 ALLOY TYPECOMPOSITION (approximate by weight) NiCrMoV low 2-4 Ni, 1-2 Cr, 0.2-0.5Mo, 0.05-0.2 V, 0.18-0.35 C, alloy steel balance Fe and incidentalimpurities. Martensitic 2-3.5 Ni, 11-14 Cr, 0.2-1.2 Mn, 1-2.5 Mo,0.2-0.5 V, stainless steel 0.05-0.2 C, balance Fe and incidentalimpurities.

Particularly preferred alloys for the HP and IP sections 12 and 14include the following: TABLE 2 ALLOY TYPE COMPOSITION (approximate bywt. %) CrMoV low alloy 0.25-0.75 Ni, 0.8-2.5 Cr, 0.5-1 Mn, 1-2.5 Mo,steel 0.2-0.35 V, 0.15-0.35 C, balance Fe and incidental impurities.Martensitic 0-1.3 Ni, 9-14 Cr, 0.1-1 Mn, 0.2-2 Mo, 0.1-0.7 V, stainlesssteel 0-3 W, 0-6 Co, 0.03-0.20 C, balance Fe and incidental impurities.Fe—Ni alloy 24-27 Ni, 13-16 Cr, 1.8-2.5 Ti, 1-1.5 Mo, 0.1-0.5 V, lessthan 2 Mn, less than 1 Si, less than 0.5 Al, less than 0.08 C, balanceFe and incidental impurities. Nickel-Base 16-20 Fe, 17-21 Cr, 2.5-3.5Mo, 4.5-5.5 Nb, Alloy 0.6-1.2 Ti, 0.2-0.8 Al, 0-1 Co, less than 0.35 Mn,less than 0.35 Si, less than 0.08 C, balance Ni and incidentalimpurities.

On the basis of combinations of the above alloys, chemistries that arebelieved to be well suited for the intermediate region 38 of theelectrode 30 include the following: TABLE 3 ALLOY COMBINATION (LPAlloy - HP/IP Alloy) COMPOSITION (approximate by wt. %) NiCrMoV—CrMoV0.25-8 Ni, 0.8-6 Cr, 0.2-1.0 Mn, 0.2-1.5 Mo, 0.05-0.35 V, 0.1-0.4 C,balance Fe and incidental impurities. NiCrMoV - Martensitic 4-8 Ni, 0-1Mn, 14-30 Cr, 1.5-5 Mo, 3-7 W, 4-12 Co, 0.2-0.35 C, balance Fe andincidental impurities. NiCrMoV—FeNi alloy 16-32 Cr, 1-4 Mn, 1.5-4 Mo,0.1-1.0 V, 0-0.5 Al, 0.5-5 Ti, 0-2 Si, 0-0.08 C, balance Ni andincidental impurities. NiCrMoV—Ni alloy 0-20 Fe, 15-40 Cr, 0-0.35 Mn,2-10 Mo, 3-12 Nb, 0-2 Al, 0-3 Ti, 0-1 Co, 0.01-0.08 C, balance (30-60)Ni and incidental impurities. Martensitic—Martensitic 0.5-8 Ni, 9-18 Cr,0-4 Mn, 0.8-4 Mo, 0.1-0.5 V, 0-0.5 Al, 0.02-0.05 N, 0.15-0.35 C, balanceFe and incidental impurities. Martensitic - FeNi alloy 0-60 Ni, 9-24 Cr,0.5-2.0 Mn, 0.5-3.0 Mo, 0-0.5 V, 0.01-0.35 C, balance Fe and incidentalimpurities. Martensitic - Alloy 718 8-12 Cr, 0.5-1.2 Mn, 0-2 Mo, 0.2-0.5V, 0-1 Co, 0.01-0.20 C, balance Fe and incidental impurities.Martensitic - Alloy 718 9-50 Cr, 0-2.0 Mn, 0-8 Mo, 0-12 Nb, 0-2 Al, 0-3Ti, 0-1 Co, 0.01-0.08 C, balance Fe and incidental impurities.

The ability to produce a monolithic multiple alloy rotor as describedabove provides various advantages. For example, fewer parts are requiredto produce a rotor as compared to prior art rotors produced by boltingor welding rotor sections together. Additional machining that wouldotherwise be required to prepare the parts for assembly is not required,and the steps of assembling and welding the parts together iseliminated. Each of these advantages reduces the cost and time requiredto produce a rotor. By eliminating the assembly requirement, the overalllength of a rotor does not need to be increased in order to accommodatea mid-span coupling, thereby minimizing related expenses such as turbineshell and site preparation costs. The ability to avoid a weldingoperation eliminates the requirement for a post-weld heat treatment,which is otherwise required in addition to the heat treatment performedafter forging. By eliminating welding, the strength level of the rotorcan be higher than that of a rotor with a welded-type constructionbecause there are no limitations on chemical composition other than thelimitations normally imposed on ingot making.

Facility costs are also reduced by eliminating the requirement forspecialized welding equipment to weld a rotor, which is particularlysignificant if a massive steam rotor is being produced. Notably,existing consumable electrode remelting (ESR and VAR) furnaces arecapable of producing rotors. Consumable electrode remelting is alsoadvantageous in that it provides a rotor that is essentially free fromsmall nonmetallic inclusions commonly found in welded construction andwhich if present can reduce load-carrying capability. Grinding andrepair welding to upgrade defective welded rotor joints will not berequired thus saving cost and production time. Little or no alloydevelopment is needed because alloys that are currently joined byassembly, welding, etc., can be employed by this invention.

As discussed above, the different chemistries of the alloys within thesections 12, 14 and 16 of the rotor forging 10 result in the formationof the transition zone 20, whose properties and shape affect thedynamics of the rotor machined from the forging 10. In particular, thetransition zone 20 affects the thermal stability of the rotor at hightemperatures, characterized by rotor centerline deflection that isdetrimental to rotor balance, turbine clearance, and high cycle fatiguelife. While consumable electrode remelting techniques are able tominimize the amount of molten metal that exists at any time in the ingotcrucible 34, and thereby limit the axial extent over which alloy mixingwill occur, a transition zone of some size and shape will be presentwithin the ingot 50 and therefore within the forging 10, particularly inview of the significant differences in the preferred alloys identifiedin Tables 1 and 2. Optimum chemistries for the electrode's intermediateregion 38 (Table 3) can have the effect of minimizing the extent of thetransition zones 20 and 60, thus reducing thermal stability attributableto the transition zone 20 in the forging 10. The chemistry of theelectrode's intermediate region 38 can also be optimized so as to limitthe extent of the transition zone 20 to assist in locating theboundaries of the chemistries of the forging regions 22 and 24.Nonetheless, forging of the multiple-alloy ingot 50 to produce the rotorforging 10 will result in the transition zone 20 having an asymmetricalshape and therefore asymmetrical material properties that negativelyaffect the dynamics of a rotor machined from the forging 10.

Accordingly, a preferred aspect of the invention is to mitigate thedetrimental effect of the transition zone 20 by minimizing the materialasymmetry within the rotor near and within the transition zone 20, thusreducing centerline deflection of the rotor when it is heated to itsoperating temperatures. For this purpose, the present inventionpreferably includes the step of altering the geometry of the machinedrotor relative to that of the rotor forging 10 by off-center machining,so that the centerline of the final machined geometry of the rotor isrelocated from that of the forging 10 to minimize the adverse effects ofthe inhomogeneity of the rotor around the transition region 20. Todetermine the extent of off-center machining required, thethree-dimensional shape of the transition zone 20 is determined using asuitable approximation technique. For example, boundary points of thetransition zone 20 within the rough-machined forging 10 can beascertained to define a plurality of axially-spaced, two-dimensionalcross-sectional shapes of the transition zone 20. These two-dimensionalcross-sectional shapes can then be used to generate thethree-dimensional shape of the transition zone 20, and finite elementmodeling (FEM) or another suitable analytical technique can be performedon the three-dimensional shape to predict the deflection of thegeometric centerline (axis of rotation) of a rotor machined from theforging 10. With this information, the centerline of the machined rotorcan be shifted by off-center machining to reduce deflection.

Various techniques could be used to develop both two andthree-dimensional shapes of the transition zone 20 within the rotorforging 10. According to one embodiment of the invention, the threedimensional shape of the transition zone 20 is approximated by measuringthe variation of chemistry at the outer surface of the rough-machinedrotor forging 10, combined with information about the likely shape ofthe transition zone 20 obtained by sectioning another rotor that wascast and forged under similar or identical conditions. With thisapproach, an axial-spaced series of two-dimensional shapes is generatedby longitudinally sectioning the similarly-processed rotor forgingspecimen along its geometric centerline, and then detecting the level ofone or more alloying constituents present in the forging specimen toidentify the boundaries separating the transition zone from the adjacentregions of the specimen. Such a technique is represented in FIG. 2, inwhich a rough-machined forging specimen 70 is depicted as containing atransition zone between two axially-aligned regions of dissimilar alloys(identified with the same reference numbers as used in FIG. 1 forconvenience). FIG. 2 represents a portion of the forging specimen 70 ashaving been cross-sectioned to expose a diametrical sectioned surface 72of the specimen 70. The chemistries of the transition zone 20 andadjacent portions of the regions 22 and 24 are then determined at anynumber of locations 74 on the sectioned surface 72. The locations 74 arealigned along radials of the forging specimen 70, with the locations 74being sufficiently close within each radial and sufficiently close alongthe axis 76 of the specimen 70 to be able to detect changes in one ormore of the alloying constituents, e.g., nickel, within each region 20,22 and 24 of the specimen 70.

In an investigation in which CrMoV and NiCrMoV alloys were used to formthe HP and LP regions 22 and 24 of a rough-machined forging specimen(e.g., similar to the forging specimen 70 of FIG. 2), a series of fivediametrical measurements, each series axially spaced about 20 to 100millimeters from the adjacent series, were made consisting of twomeasurements at the outer surfaces of the forging specimen, at the axisof the specimen, and at radial midpoints between the outer surfaces andthe axis. FIG. 4 is a graph plotting the nickel content (wt. %) versusaxial location for each measured radial location of the specimen. Togenerate two-dimensional shapes from the data, an optimum curve wasfound that would fit the data points. While a continuous curve could beused, a linear three-step function is plotted in FIG. 4 by grouping thedata points into three groups according to nickel content, one groupbeing in the range of about 0.5% to about 2.25%, the second in the rangeof 2.25% to 3.2%, and the third in the range of 3.2% to 3.5%. Each groupof data was fitted with a straight line using least square estimation onthe condition that the two lines must share a common point at 2.25% and3.2%.

After the function was determined, the axial location of 0.5%, 2.25%,3.2%, and 3.5% nickel levels were calculated at the outer surfacelocations, the mid-radial locations, and the axis location. The axiallocation at the step transition was calculated because a step functionto cover a range of 0.5% to 3.5% nickel content was used. The 0.5% and3.5% nickel levels were chosen as generally corresponding to the levelsof nickel for the CrMoV and NiCrMoV alloys, respectively, forming theregions of the specimen outside the transition zone, and thereforeindicative of the boundaries of the transition zone. In the presentexample where each radial series consisted of five measurements, eachboundary (nickel levels of 0.5% or 3.5%) of the transition zone and eachsubdivided zone was located with at least five measured points. Severalmethods are available for fitting each of the five measurements to atwo-dimensional curve. For example, different orders of polynomial orcubic spline curve fitting could be used. FIG. 5 illustrates the use ofa cubic spline curve fitting, which is capable of generating acontinuous, smooth curve that passes through each set of measurementscorresponding to the same nickel level.

As evident from FIG. 5, each two-dimensional curve has an asymmetricshape relative to the axis of the specimen. As previously noted, thisasymmetry is largely attributable to the forging operation, and would bedetrimental to the thermal stability of a rotor machined from thespecimen. The asymmetry of each two-dimensional is such that any attemptto establish three-dimensional axial boundaries (corresponding to nickellevels from 0.5% to 3.5%) by simply rotating the two halves of the curveon each side of the centerline at each nickel level would generate twovolcano-shaped three-dimensional images of each boundary. To avoid thisresult, a suitable approach is to rotate each half of the curve through180 degrees, gradually changing the shape of the curve during rotationso that the curve acquires the shape of the other half of the curve atthe completion of the 180-degree rotation, so that the rotated curvescoincide with each other when they meet. This can be achieved byassuming a two-dimensional curve function z=f(r), where r is the valuefor the radial direction and z is the value for the axial direction of agiven point. Rotating a two-dimensional curve to generate athree-dimensional surface adds the variable θ, which is the variable forthe circumferential direction. Using this technique, the equation to useis:z(r,θ)=z ₁ cos²(θ/2)+z ₂ sin²(θ/2)=f ₁(r,θ)cos²(θ/2)+f ₁(−r,θ)sin²(θ/2)

FIG. 6 shows the two-dimensional curve defining the 0.5% nickel boundaryof FIG. 5 as its shape is modified when rotated through angles of 45,90, 135 and 180 degrees in accordance with the above equation. From thethree-dimensional contour defined by the curves of FIG. 6, a finiteelement model can be generated and used to predict the centerlinedeflection of a rotor machined from a similar rotor forging and thenheated to some elevated temperature, e.g., a temperature to which therotor would be subjected during its operation. More particularly, athree-dimensional approximation of the shape of the transition zonewithin a similarly-processed rotor forging having the same multiplealloy composition can be predicted by measuring the variation ofchemistry at the outer surface of the rotor forging, and then utilizingthe shape of the transition zone obtained from the analysis of thesectioned forging specimen. FIG. 7 shows the results of such analysisusing a temperature of about 1150° F. (about 620° C.), and evidencesthat the predicted deflection profile is very close to the actualdeflection profile of a rotor machined from a rotor forging produced bythe same process as the forging specimen.

FIG. 3 represents an alternative to the chemical analysis approachdescribed above, in which the boundary points of the transition zone 20are ascertained by ultra-sonically examining the rough-machined forging10. Such an approach is disclosed in copending and commonly-as-signedU.S. Patent Application Serial No. {Attorney Docket No. 132847},incorporated herein by reference. In an investigation in which thisapproach was implemented in the present invention, an ultrasonictransducer 78 was placed against the outer surface of the forging 10,and ultrasonic energy was transmitted through the forging 10 along thetransition zone 20 to detect changes in the response of the forgingmaterial to ultrasonic energy. As evidenced by FIG. 8, ultrasonicinspection of the forging 10 produced a noise pattern corresponding tovariations in the metallurgical characteristics within the forging 10,such as differences in grain size attributable to the changes inchemistry between the transition zone 20 and the adjacent regions 22 and24. After filtering the peak noise at each scan line, the data is fittedto a circular or elliptical contour 26 representative of the expectedcross-sectional (two-dimensional) shape of each transition zoneboundary. FIG. 9 represents a three-dimensional image 27 generated bycombining multiple contours 26 taken along the length of the forging 10.

Using the image of FIG. 9, a three-dimensional mathematical model wasgenerated of the transition zone 20 by axially sectioning eachtransition zone contour 26 of the image with planes intersecting at thegeometric centerline 18 of the forging 10. FIG. 10 represents one of theplanes 28 intersecting one of the contours 26 at two points. Assumingthe transition zone contour 26 has a radius r_(o), the center of thecontour 26 is offset from the forging centerline 18 a distance A at anangle θ_(o), and the plane 28 at an angle θintersects the contour 26 todefine radial distances r₁ and r₂ from the centerline 18, the followingequations can be used to find the radial distances r₁ and r₂.r ₁ =A cos(θ_(o)−θ)+(r _(o) ² −A ² sin²(θ_(o)−θ))^(1/2)r ₂=−(−A cos(θ_(o)−θ)+(r _(o) ² −A ² sin²(θ_(o)−θ))^(1/2))

If the total number of transition zone contours 26 is N_(c), then amaximum of 2N_(c) number of intercept points are obtained by cutting thetransition zone 20 with a plane 28 of at an angle θ, and a maximum of2N_(c)N_(p) intercept points can be obtained if the transition zone 20is cut with N_(p) number of planes 28. These intercept points can thenbe curve fit using polynomial or cubic spline curve fitting techniques.If a polynomial curve fitting technique is used, different orders arepreferably tried until a good curve fit of the data (e.g., R² above 95%)is obtained. The peak position of each polynomial at different anglesshould also be consistent, i.e., all curves end up with the same peak.If a cubic spline fitting technique is used, an additional data point atthe peak location is preferred. A set of curve fit coefficients from theprevious step can then be used to interpolate points betweenintercepting points on adjacent scans within two adjacent angles, sothat a three-dimensional model for the transition zone 20 can bemathematically constructed.

In the investigation described above, the rotor forging 10 was producedfrom an ESR ingot in which the LP and IP/HP alloys were NiCrMoV andCrMoV, respectively. Assumptions for interpreting the ultrasonic dataincluded a transition in the chemistry (and thus property) distributionoccurred at a nickel content of about 3.0 weight percent, and that theultrasonic inspection produced a noise pattern corresponding tovariations in the metallurgical characteristics that occurred at the 3.0wt. % nickel location. From the two-dimensional contours 26 andthree-dimensional contour 27 defined in the manner described above, athree-dimensional finite element model was generated and used to predictthe centerline deflection of a rotor machined from the rotor forging 10and then heated to some elevated temperature, e.g., a temperature towhich the rotor would be subjected during its operation. FIG. 11 showsthe results of such analysis using a furnace with three differenttemperature zones (about 400, 800 and 1000° F. (about 200, 425, and 540°C.)) to simulate the variation of temperature along the length of arotor during operation, and evidences that the predicted deflectionprofile was very close to the actual deflection profile of the machinedrotor. This model was also able to predict the rotor centerlinedeflection before and after removing (gashing) the material betweenwheels of the rotor.

Finally, to reduce the amount of centerline deflection from thatpredicted through the above-described modeling techniques, a new rotoraxis is identified that is more centrally located within the transitionzone 20 than the forging centerline 18, while still within machiningtolerances. The forging (or, more typically, a rough-machined geometryformed by rough machining the forging) can then be off-center machinedto establish the new axis as the geometric centerline (and therefore theaxis of rotation) of the final machined rotor geometry. The methodologyinvolves translating the coordinates of the rough-machined forging fromthe geometric centerline of the forging to the new axis, which is at ornear the center of a three-dimensional approximation of the transitionzone 20.

To explain the methodology, reference is made to FIG. 12 which shows themachined outer profile of a rotor 90 at one axial position. Forillustrative purposes, the rotor 90 is indicated as having an as-forgedgeometric centerline 18, corresponding to the rough-machined forging 10of FIG. 3. The location for the new rotor axis, more centrally locatedwithin the rotor 90 (and approximately centrally located within thetransition zone 20 of the forging 10), is identified with referencenumber 98. The offset of the new rotor axis 98 relative to the forgingcenterline 18 is exaggerated to assist in clarifying the methodology. InFIG. 12, the new rotor axis 98 is offset a distance A and on an angleθ_(o) from the forging center-line 18. A point P is located a distancer_(o) and at an angle αrelative to the forging centerline 18, and adistance r and at an angle θ relative to the new rotor axis 98. Thecoordinates (r,θ) in the coordinate system with its center at the axis98 can be obtained with the following equations:r _(o)=[(r ² +A ²−2Ar cos(θ−θ_(o))]^(1/2)tan α=(r sinθ−A sin θ_(o))/(r cosθ−A cosθ_(o))

As atan(x) is in the range of −90 to +90 degrees, and the angle in acylindrical coordinate system is in the range of −180 to +180 degrees,the following table can be used to obtain angle α. α = (rcosθ −acosθ₀) > 0 (rcosθ − acosθ₀) < 0 (rsinθ − asinθ₀) > 0$a\quad\tan\frac{{r\quad\sin\quad\theta} - {a\quad\sin\quad\theta_{0}}}{{r\quad\cos\quad\theta} - {a\quad\cos\quad\theta_{0}}}$${a\quad\tan\frac{{r\quad\sin\quad\theta} - {a\quad\sin\quad\theta_{0}}}{{r\quad\cos\quad\theta} - {a\quad\cos\quad\theta_{0}}}} + {180{^\circ}}$(rsinθ − asinθ₀) < 0$a\quad\tan\frac{{r\quad\sin\quad\theta} - {a\quad\sin\quad\theta_{0}}}{{r\quad\cos\quad\theta} - {a\quad\cos\quad\theta_{0}}}$${a\quad\tan\frac{{r\quad\sin\quad\theta} - {a\quad\sin\quad\theta_{0}}}{{r\quad\cos\quad\theta} - {a\quad\cos\quad\theta_{0}}}} - {180{^\circ}}$This table can be summarized by one common equation as:$\alpha = {{{Atan}\frac{{r\quad\sin\quad\theta} - {A\quad\sin\quad\theta_{0}}}{{r\quad\cos\quad\theta} - {A\quad\cos\quad\theta_{0}}}} + {90^{O} \times \left( {1 - \frac{{r\quad\cos\quad\theta} - {A\quad\cos\quad\theta_{0}}}{{{r\quad\cos\quad\theta} - {A\quad\cos\quad\theta_{0}}}}} \right) \times \quad\frac{{r\quad\sin\quad\theta} - {A\quad\sin\quad\theta_{0}}}{{{r\quad\sin\quad\theta} - {A\quad\sin\quad\theta_{0}}}}}}$

By translating the rotor geometry to the new coordinate system with theoff-center magnitude A and the off-set angle α from the forgingcenterline 18, all points based on a three-dimensional model generatedas described above are translated to the new coordinate system with itscenter at centerline 98. In this manner, any axial position in thetransition zone of the forging 10 can be machined using the newcoordinate system so the axis of rotation of the machined rotor 90coincides with the axis 98, and therefore is more centrally located withrespect to the three-dimensional approximation of the shape of thetransition zone 20 of the rough-machined forging 10. In so doing, theaxis of rotation of the machined rotor 90 is also more centrally locatedwith respect to the material properties of the forging 10 than was thegeometric centerline 18 of the forging 10. As a result, the machinedrotor 90 is able to exhibit less deflection when heated and rotatedabout its axis of rotation 98 than would the machined rotor if it hadbeen machined so that its axis of rotation coincided with the geometriccenterline 18 of the forging 10.

The procedure just described can be repeated at several positions alongthe length of the transition zone. The results of the severalcalculations can be analyzed using finite element techniques inconjunction with optimization techniques to select the best choice ofoff-center machining. Based on the new rotor axis 98 identified by theabove process, thermal analysis can be performed to obtain thetemperature gradient of the entire rotor 90 to permit recalculating ofthe predicted rotor centerline deflection. To find the optimum locationof the new rotor axis 98, the largest possible off-center machinetolerances can be calculated based on the difference between thediameter of the rotor 90 at its rough-machined geometry and the diameterof the rotor 90 at its final machined geometry. Using the off-centermachine distance A and the off-center angle α as variables, and themaximum rotor centerline deflection as response, a series of runs andanalysis-evaluation-modification cycles can be performed with randomvariable inputs for A and α. A response curve can then be generatedusing least square curve fitting between data points to identify theminimum centerline deflection within the allowed range for off-centershift. In one investigation implementing the above-describedmethodology, relocating the rotor axis 98 a distance of about 12 mm fromthe original rough-machined center-line 18 reduced centerline deflectionby about 65%.

While the invention has been described in terms of one or moreparticular embodiments, it is apparent that other forms could be adoptedby one skilled in the art. Therefore, the scope of the invention is tobe limited only by the following claims.

1. A process for producing a rotor, the process comprising the steps of:casting an ingot to have at least first and second ingot regions axiallyaligned within the ingot, the first and second ingot regions beingformed of different alloys that intermix during casting to define atransition zone between the first and second ingot regions, thetransition zone having a composition that differs from and variesbetween the first and second ingot regions; forging the ingot to producea rotor forging containing first and second forging regions and atransition zone therebetween corresponding to the first and second ingotregions and the transition zone of the ingot such that the first andsecond forging regions are formed of the different alloys and thetransition zone of the rotor forging has a composition that differs fromand varies between the first and second forging regions, the first andsecond forging regions and the transition zone therebetween beingaxially aligned along a geometric centerline of the rotor forging, thetransition zone of the rotor forging being asymmetrical about thegeometric centerline of the rotor forging following the forging step;identifying an axial line through the rotor forging that is morecentrally located with respect to material properties of the rotorforging than the geometric centerline of the rotor forging; and thenmachining the rotor forging to produce a machined rotor containing firstand second rotor regions and a transition zone therebetweencorresponding to the first and second forging regions and the transitionzone of the rotor forging such that the first and second rotor regionsare formed of the different alloys and the transition zone of themachined rotor has a composition that differs from and varies betweenthe first and second rotor regions, wherein the machining step isperformed so that the axial line of the rotor forging defines an axis ofrotation of the machined rotor, the machined rotor exhibiting lessdeflection when heated to an elevated temperature than would themachined rotor if machined so that the geometric centerline thereofdefined the axis of rotation of the machined rotor and the machinedrotor were heated to the elevated temperature.
 2. The process accordingto claim 1, further comprising heat treating the first and secondforging regions to different temperatures after the forging step andbefore the machining step.
 3. The process according to claim 1, furthercomprising the steps of: producing a rotor forging specimen inaccordance with the casting and forging steps of claim 1, whereby therotor forging specimen contains first and second specimen regions and atransition zone therebetween, the first and second specimen regions areformed of the different alloys, the transition zone of the rotor forgingspecimen has a composition that differs from and varies between thefirst and second specimen regions, and the first and second specimenregions and the transition zone there-between are axially aligned alonga geometric centerline of the rotor forging specimen; ascertainingboundary points of the transition zone within the rotor forging specimento define a plurality of two-dimensional cross-sectional shapes of thetransition zone; using the plurality of two-dimensional cross-sectionalshapes to produce a three-dimensional approximation of the shape of thetransition zone within the rotor forging specimen; and using thethree-dimensional approximation of the shape of the transition zonewithin the rotor forging specimen to identify the axial line of therotor forging.
 4. The process according to claim 3, wherein the step ofidentifying the axial line of the rotor forging comprises determining atan outside surface of the rotor forging the level of at least onealloying constituent of at least one of the different alloys of thefirst and second specimen regions.
 5. The process according to claim 1,wherein the step of identifying the axial line through the rotor forgingcomprises producing a three-dimensional approximation of the shape ofthe transition zone within the rotor forging by ultrasonically examiningthe rotor forging.
 6. The process according to claim 1, wherein thefirst rotor region is located within a high pressure region of themachined rotor and is formed from an alloy chosen from the groupconsisting of CrMoV low alloy steels, martensitic stainless steelscontaining about 9 to about 14 weight percent chromium, Fe—Ni alloys,and nickel-base alloys, and the second rotor region is located within alow pressure region of the machined rotor and is formed from an alloychosen from the group consisting of NiCr—MoV low alloy steels andmartensitic stainless steels containing about 11 to about 14 weightpercent chromium.
 7. The process according to claim 1, wherein thecasting step comprises a consumable electrode remelting technique anduses an electrode having a first section and a second section contactingthe first section, the first section corresponding in composition to thefirst ingot region and the second section corresponding in compositionto the second ingot region.
 8. The process according to claim 1, whereinthe casting step comprises a consumable electrode remelting techniqueand uses an electrode having a first section corresponding incomposition to the first ingot region, a second section corresponding incomposition to the second ingot region, and an intermediate sectionbetween the first and second sections and having a composition thatdiffers from the compositions of the first and second sections of theelectrode.
 9. The process according to claim 8, wherein the compositionof the first section of the electrode is a CrMoV low alloy steel, thecomposition of the second section of the electrode is a NiCrMoV lowalloy steel, and the composition of the intermediate section of theelectrode consists of, by weight, about 0.25 to about 8% nickel, about0.8 to about 6% chromium, about 0.2 to about 1.0% manganese, about 0.2to about 1.5% molybdenum, about 0.05 to about 0.35% vanadium, about 0.1to about 0.4% carbon, the balance iron and incidental impurities. 10.The process according to claim 8, wherein the composition of the firstsection of the electrode is a martensitic stainless steel containingabout 11 to about 14 weight percent chromium, the composition of thesecond section of the electrode is a NiCrMoV low alloy steel, and thecomposition of the intermediate section of the electrode consists of, byweight, about 4 to about 8% nickel, about 14 to about 30% chromium, upto about 1% manganese, about 1.5 to about 5% molybdenum, about 3 toabout 7% tungsten, about 4 to about 12% cobalt, about 0.2 to about 0.35%carbon, the balance iron and incidental impurities.
 11. The processaccording to claim 8, wherein the composition of the first section ofthe electrode is a Fe—NI alloy, the composition of the second section ofthe electrode is a NiCrMoV low alloy steel, and the composition of theintermediate section of the electrode consists of, by weight, about 16to about 32% chromium, about 1 to about 4% manganese, about 1.5 to about4% molybdenum, about 0.5 to about 5% titanium, up to about 0.5%aluminum, about 0.1 to about 1.0% vanadium, up to about 2% silicon, upto about 0.08% carbon, the balance nickel and incidental impurities. 12.The process according to claim 8, wherein the composition of the firstsection of the electrode is a nickel-based alloy, the composition of thesecond section of the electrode is a NiCrMoV low alloy steel, and thecomposition of the intermediate section of the electrode consists of, byweight, about 15 to about 40% chromium, up to about 0.35% manganese,about 2 to about 10% molybdenum, about 3 to about 12% niobium, up toabout 3% titanium, up to about 2% aluminum, up to about 1% cobalt, up toabout 20% iron, about 0.01 to about 0.08% carbon, the balance nickel andincidental impurities.
 13. The process according to claim 8, wherein thecomposition of the first section of the electrode is a martensiticstainless steel containing about 9 to about 14 weight percent chromium,the composition of the second section of the electrode is a martensiticstainless steel containing about 11 to about 14 weight percent chromium,and the composition of the intermediate section of the electrodeconsists of, by weight, about 0.5 to about 8% nickel, about 9 to about18% chromium, up to about 4% manganese, about 0.8 to about 4%molybdenum, about 0.1 to about 0.5% vanadium, up to about 0.05%aluminum, about 0.02 to about 0.05 nitrogen, about 0.15 to about 0.35%carbon, the balance iron and incidental impurities.
 14. The processaccording to claim 8, wherein the composition of the first section ofthe electrode is a Fe—Ni alloy, the composition of the second section ofthe electrode is a martensitic stainless steel containing about 9 toabout 14 weight percent chromium, and the composition of theintermediate section of the electrode consists of, by weight, up toabout 60% nickel, about 9 to about 24% chromium, about 0.5 to about 2%manganese, about 0.5 to about 3% molybdenum, up to about 0.5% vanadium,about 0.10 to about 0.35% carbon, the balance iron and incidentalimpurities.
 15. The process according to claim 8, wherein thecomposition of the first section of the electrode is Alloy 718, thecomposition of the second section of the electrode is a martensiticstainless steel containing about 9 to about 14 weight percent chromium,and the composition of the intermediate section of the electrodeconsists of, by weight, about 8 to about 12% chromium, about 0.5 toabout 1.2% manganese, up to about 2% molybdenum, about 0.2 to about 0.5%vanadium, up to about 1% cobalt, about 0.01 to about 0.2% carbon, thebalance iron and incidental impurities.
 16. The process according toclaim 8, wherein the composition of the first section of the electrodeis Alloy 718, the composition of the second section of the electrode isa martensitic stainless steel containing about 9 to about 14 weightpercent chromium, and the composition of the intermediate section of theelectrode consists of, by weight, about 9 to about 50% chromium, up toabout 2% manganese, up to about 8% molybdenum, up to about 12% niobium,up to about 2% aluminum, up to about 3% titanium, up to about 1% cobalt,about 0.01 to about 0.08% carbon, the balance iron and incidentalimpurities.
 17. A process for producing a monolithic steam turbinerotor, the process comprising the steps of: casting an ingot using aconsumable electrode remelting technique to have at least first andsecond ingot regions axially aligned within the ingot, the first andsecond ingot regions being formed of different alloys that inter-mixduring casting to define a transition zone between the first and secondingot regions, the transition zone having a composition that differsfrom and varies between the first and second ingot regions; forging theingot to produce a rotor forging containing first and second forgingregions and a transition zone therebetween corresponding to the firstand second ingot regions and the transition zone of the ingot such thatthe first and second forging regions are formed of the different alloysand the transition zone of the rotor forging has a composition thatdiffers from and varies between the first and second forging regions,the first and second forging regions and the transition zonetherebetween being axially aligned along a geometric centerline of therotor forging; ascertaining boundary points of the transition zonewithin the rotor forging to define a plurality of two-dimensionalcross-sectional shapes of the transition zone; using the two-dimensionalcross-sectional shapes of the transition zone to produce athree-dimensional approximation of the shape of the transition zone;using the three-dimensional approximation to predict deflection of thegeometric centerline of the rotor forging if the rotor forging were tobe heated to an elevated temperature; identifying an axial line throughthe rotor forging that is more centrally located with respect tomaterial properties of the rotor forging and the three-dimensionalapproximation of the shape of the transition zone than the geometriccenterline of the rotor forging; and then machining the rotor forging toproduce a machined monolithic rotor in which the axial line of the rotorforging defines an axis of rotation of the machined monolithic rotor,the machined monolithic rotor exhibiting less deflection at the elevatedtemperature than the deflection predicted for the rotor forging at theelevated temperature.
 18. The process according to claim 17, wherein theboundary points of the transition zone are ascertained by longitudinallysectioning a specimen of the rotor forging along the geometriccenterline thereof and determining the level of at least one alloyingconstituent of at least one of the different alloys of the first andsecond forging regions.
 19. The process according to claim 17, whereinthe boundary points of the transition zone are ascertained byultrasonically examining the rotor forging along the geometriccenterline thereof to determine variations in metallurgicalcharacteristics of the first and second forging regions and thetransition zone therebetween.
 20. The process according to claim 17,wherein the three-dimensional approximation of the shape of thetransition zone of the rotor forging is asymmetrical about the geometriccenterline of the rotor forging.
 21. The process according to claim 17,wherein as a result of the forging and machining steps the first ingotregion defines a first rotor region located within a high pressureregion of the machined monolithic rotor, and the second ingot regiondefines a second rotor region located within a low pressure region ofthe machined monolithic rotor, the first rotor region being formed froman alloy chosen from the group consisting of Cr—MoV low alloy steels,martensitic stainless steels containing about 9 to about 14 weightpercent chromium, Fe—Ni alloys, and nickel-base alloys, and the secondrotor region being formed from an alloy chosen from the group consistingof NiCrMoV low alloy steels and martensitic stainless steels containingabout 11 to about 14 weight percent chromium.
 22. The process accordingto claim 17, wherein the casting step comprises using an electrodehaving a first section corresponding in composition to the first ingotregion, a second section corresponding in composition to the secondingot region, and an intermediate section between the first and secondsections and having a composition that differs from the compositions ofthe first and second sections of the electrode.
 23. The processaccording to claim 17, further comprising the step of simultaneouslyheat treating the first and second forging regions at differenttemperatures.
 24. A monolithic rotor formed by machining a rotorforging, the monolithic rotor comprising first and second rotor regionsaxially aligned within the monolithic rotor and a transition zonetherebetween, the first and second rotor regions being formed ofdifferent alloys and the transition zone having a composition thatdiffers from and varies between the first and second rotor regions, thetransition zone having a three-dimensional shape about a centerline ofthe rotor forging, the monolithic rotor having an axis of rotation thatis more centrally located with respect to the transition zone than thecenterline of the rotor forging.
 25. The monolithic rotor according toclaim 24, wherein the axis of rotation of the monolithic rotor is morecentrally located with respect to material properties of the monolithicrotor than the centerline of the rotor forging.
 26. The monolithic rotoraccording to claim 24, wherein the first rotor region is located withina high pressure region of the monolithic rotor and is formed from analloy chosen from the group consisting of CrMoV low alloy steels,martensitic stainless steels containing about 11 to about 14 weightpercent chromium, Fe—Ni alloys, and nickel-base alloys, and the secondrotor region is located within a low pressure region of the monolithicrotor and is formed from an alloy chosen from the group consisting ofNiCrMoV low alloy steels and martensitic stainless steels containingabout 11 to about 14 weight percent chromium.
 27. The monolithic rotoraccording to claim 24, wherein the rotor is a steam turbine rotor.
 28. Asteam turbine in which the monolithic rotor according to claim 27 isinstalled.
 29. The monolithic rotor according to claim 24, wherein therotor is a gas turbine engine rotor.
 30. A gas turbine engine in whichthe monolithic rotor according to claim 29 is installed.
 31. Themonolithic rotor according to claim 24, wherein the rotor is a jetengine rotor.
 32. A jet engine in which the monolithic rotor accordingto claim 31 is installed.